In certain applications, it may be desirable that operating systems which are designed for stand-alone operation are instead installed for use in tandem. This may be desirable to provide greater reliability or efficiency. Or, if greater capacity is needed, it may be less costly to add or combine a second similar or identical system to the first rather than to install a larger one. This is certainly true where replacing already-installed system may result in scrapping the old one. And where greater manufacturing volume exists for the smaller system, installing two identical units allows the costs for the installation to reflect returns to scale not possible with the larger system. As an example, it is sometimes desirable to install two identical furnaces or air conditioning units whose total capacity will handle the thermal load rather than a single unit of the given capacity. In this situation, two furnaces may more closely match the capacity of an associated air conditioning unit, or vice versa if two air conditioning units are employed.
Running two operating systems in tandem however, is not always easy to accomplish. It is sometimes desirable for controlling such a tandem system in a consistent fashion that the two units operate synchronously, with each step in the operating sequence of each starting at very nearly the same time. Turning to the temperature control example once more, in a tandem forced air heating or cooling system, if power is applied to one blower before the other, the air from the operating blower is forced through the other blower. This reverse air flow will cause the unpowered blower to rotate backwards. The result for the induction motor commonly employed to drive these fans is that it will continue to run backwards when it finally receives operating power. It is possible to place an overriding brake on each of the motors to prevent this, but the additional mechanical components reduce the reliability of the system. Therefore, in this case, it is desirable to synchronize the operation of each system to cause the blowers to both start at very nearly the same time to prevent the unpowered fan from running backward. This timing requirement can be achieved only by some type of communication between the two systems' controllers.
It is well known now to use small microcontrollers to control the operation of various types of operating systems. In the temperature control example, microcontrollers now commonly control the sequencing of individual steps in the operation of air conditioning units and furnaces used for cooling and heating the spaces in buildings of various types. These microcontrollers are very fast and reliable and can therefore provide many more functions for the operating system involved than were possible when mechanical timers were used for controlling and sequencing these functions. At the same time, the greater capability of these microcontrollers allows the operation of these systems to be supervised more closely than was previously possible. This has resulted in safer and more efficient operation for whatever type of operating system is involved. One capability which is inherent in microcontrollers is that of fast and reliable communication with an external device such as a similar microcontroller. It is therefore appealing to use this communication capability to address the problem of synchronizing the operation of tandem operating systems. There are now in existence, three wire communication systems which are robust in terms of reliability and noise insensitivity.
One problem which these systems have had heretofore is that if one of the systems should lose power or the communication paths between the systems should become disconnected, the other may not find it easy to discover this situation. It is desirable that the communication system allow for at least some self-checking for detecting loss of power to one of the control units or disconnection of the communication system's conductors. Since one system has by definition lost power in this situation, how to determine loss of power in that system by the other is not obvious.